Gakken SX-150 MKII MIDI-IN Mod

MIDI-IF Build Notes

It’s annoying sometimes having an interesting synthesizer like the Gakken SX-150 MKII in my rack when it’s not MIDI capable. My solution is to break out the soldering iron and build a usable MIDI-IN jack using the MIDI-IF Circuit Board Kit put together by Beatnic.jp.

I’ll be covering step-by-step instructions on how to build the MIDI-IF circuit board kit and providing build notes for reference. I suggest researching the available resources found on the Beatnic.jp website before beginning. Links are provided in the Build Resources section below.

You’ll need a better than beginner level of electronics knowledge to complete this build, as it is moderately complicated. I wouldn’t recommend beginners start out their electronics hobby on a project like this.

The build requires an understanding of how to read an electronics diagram, how to identify electronics components and a decent skill level at through-hole soldering. I won’t be teaching any of these things in this Lab and will be assuming you have an understanding of these skills as I present my instructions.

Step-by-step InstructionsBefore you break out the soldering iron, it’s always good practice to identify all of your components. The MIDI-IF kit came with a parts list and I’ve provided a Bill Of Material for you to review. It’s probably a good idea to print these items and the available photos for reference.

I like to sort and identify the components on a folded sheet of paper so they can be easily located when I’m ready for them, but this is a personal choice and workflow.

The other thing I like to do is skip trying to read the color bands on the resistors and use a multimeter to identify their Ohms resistance value. This was a very useful strategy while building this project because there are several components in the kit with a number “47” in their value.

Part 1 – Cut The PCB Traces

1.1 – Use an X-Acto knife to very carefully cut the copper trace lines on the PCB as shown in the diagram and photo. Do this on a firm and flat surface making sure to only cut the two traces that are identified. The trace is tiny, so take your time a do it right.

1.2 – Carefully heat the trace area between your cuts with the tip of your soldering iron for a few seconds, and then gently scrape away the heated trace section with your X-Acto knife.

1.3 – Check to make sure the traces have been completely disconnected from the circuit paths by using a multimeter in continuity test mode. We’re going to create bridge jumpers in this area later on, so we need to make absolutely positive the original traces have been disconnected.

2.2 – Insert the jumper wires. I used the trimmed legs from one of the resistors to create the three jumper wire pieces. Refer to the diagram image to identify where the jumpers wires need to be placed. Solder into place.

2.7 – These parts have polarity and must be mounted in the correct direction. Check the diagram and photo before you continue. Take note of the + Anode marks printed on the circuit board. Mount the components. The negative legs are indicated by a stripe: D1-1N4148, C4-1uF, C5-0.47uF, C8-1uF, U5-78L05. Solder into place.

2.8 – Mount capacitors: C2-4.7n, C6-22n, C7-22n. Solder into place.

2.9 – Mount DIP components and trim pot: U1-ATTiny2313 (20pins), U2-6N138 (8pins), U3-LM358 (8pins), Trim-Pot. Don’t install the IC chips yet because there’s more soldering to do. We’ll want to make sure the chips are inserted into the DIP slots in the correct orientation after all of the soldering has been completed.

2.10 – Check all of your solder joints. Do you still have components to solder?

2.11 – Construct the Jumper pin bridges. This part is a bit tricky and you’ll need to improvise a bit. The trace lines we cut earlier need to be bridged at this point. I had to heat up one of the DIP solder point pads with the soldering iron and pass a bridge wire through a couple of holes using the same technique. This is BAD circuit board design, but it can be dealt with… so don’t get too frustrated.

Part 3 – Adding The Wiring Components And IC Chips

I’m using 22-gauge stranded hook-up wire for the connections. You can follow my wiring color code scheme if you have a variety of connect wire colors available or keep it simpler if you need too. The point is to identify your wiring in some way, even if it’s with pieces of masking tape and a Sharpie pen.

At some point, you’ll probably want to put this board into an enclosure, or maybe modify things. Even though the external wiring of the MIDI-IF circuit board isn’t very complicated, it’s good practice to be consistent with your wiring practices and always keep some notes. Chances are, you’ll be doing some troubleshooting at some point.

3.1 – Add the MIDI-IN connector. Wires are soldered between MIDI connector PIN4 to the MIDI4 location and MIDI connector PIN 5 to the MIDI5 location on the printed circuit board.

3.2 – Add the Potentiometers. I refer to the solder points on the pots as PIN1, PIN2, PIN3.

3.3 – Add the battery clip.

3.4 – Insert the IC chips into the appropriate DIP sockets. Refer to the build of materials and circuit diagram for the proper locations.